1. Field of the Invention
The present invention relates to an organic light emitting display, and a method of fabricating the same.
2. Discussion of Related Art
Generally, electroluminescence displays, which are self-luminous displays, electrically excite fluorescent materials to emit light. These electroluminescence displays have generated significant interest as next-generation displays due to advantages such as low voltage operation, a very slim form factor, a wide viewing angle, a high speed response, etc., which are advantages that generally cannot be achieved in liquid crystal displays.
Electroluminescence displays include inorganic light emitting displays and organic light emitting displays, depending on whether the material that makes up the light emitting layer is an inorganic material or an organic material.
Organic light emitting displays have an organic layer in a pattern (e.g., a predetermined pattern) formed on a glass substrate or other transparent insulating substrate. An anode electrode layer and a cathode electrode layer are formed on an upper part and a lower part of the organic layer, respectively. The organic layer includes organic compounds that form a light emitting layer.
In the organic light emitting display configured as described above, a positive voltage and a negative voltage are applied to an anode electrode and a cathode electrode, respectively, such that holes move from the anode electrode with the positive voltage to the light emitting layer via a hole transportation layer, and electrons move from the cathode electrode with the negative voltage to the light emitting layer via an electron transportation layer. Accordingly, the electrons and the holes are recombined in the light emitting layer to generate excitons, and the excitons change from an excited state to a ground state, so that fluorescent molecules of the light emitting layer emit light to form an image.
FIG. 1 is a circuit diagram showing a conventional pixel of an organic light emitting display. Referring to FIG. 1, the pixel 4 of the organic light emitting display includes a pixel circuit 2 coupled to an organic light emitting diode OLED, a data line Dm, and a scan line Sn to control light emission of the organic light emitting diode OLED.
An anode electrode of the organic light emitting diode OLED is coupled to the pixel circuit 2 and a cathode electrode of the organic light emitting diode OLED is coupled to a second power supply ELVSS. The organic light emitting diode OLED emits light corresponding to a current supplied from the pixel circuit 2.
The pixel circuit 2 controls the amount of current supplied to the organic light emitting diode OLED in accordance with to a data signal supplied to the data line Dm when a scan signal is supplied to the scan line Sn.
To this end, the pixel circuit 2 includes a second transistor M2 coupled between a first power supply ELVDD and the organic light emitting diode OLED; a first transistor M1 coupled between the second transistor M2 and the data line Dm, and coupled to the scan line Sn to receive the scan signal; and a storage capacitor Cst coupled between a first electrode and a gate electrode of the second transistor M2.
A gate electrode of the first transistor M1 is coupled to the scan line Sn and a first electrode of the first transistor M1 is coupled to the data line Dm. A second electrode of the first transistor M1 is coupled to one terminal of the storage capacitor Cst.
Herein, the first electrode is one of a source electrode and a drain electrode, and the second electrode is the other one of the source electrode and the drain electrode. For example, if the first electrode is the source electrode, the second electrode is the drain electrode. The first transistor M1 coupled to the scan line Sn and the data line Dm is turned on when the scan signal is supplied from the scan line Sn to supply the data signal supplied from the data line Dm to the storage capacitor Cst. At this time, the storage capacitor Cst charges a voltage corresponding to the data signal.
The gate electrode of the second transistor M2 is coupled to one terminal of the storage capacitor Cst and the first electrode of the second transistor M2 is coupled to the other terminal of the storage capacitor Cst and the first power supply ELVDD. The second electrode of the second transistor M2 is coupled to the anode electrode of the organic light emitting diode OLED.
The second transistor M2 controls the amount of current flowing from the first power supply ELVDD to the second power supply ELVSS via the organic light emitting diode OLED, where the amount of current corresponds to a voltage value stored in the storage capacitor Cst. According to this configuration, the organic light emitting diode OLED generates light corresponding to the amount of current supplied from the second transistor M2.
The organic light emitting display may be a top emission type, a bottom emission type, or both, according to a light emitting direction. Recently, as flat displays have become bigger in size, the adoption of the top emission type has been preferred.
In top emission type organic light emitting displays, the lower part of the organic layer is coupled to the anode electrode and the upper part of the organic layer transmitting light is coupled to the cathode electrode.
In some organic light emitting displays, the cathode electrode includes a transflective layer (i.e., a layer that both transmits and reflects light), and in the case of the top emission type, a cathode electrode having a low work function and a transflective characteristic is frequently utilized.
Accordingly, the cathode electrode may be very thin to implement the transflective characteristic. In this case, the cathode electrode has a high resistance.
In particular, in the case where the organic light emitting display is driven with current, a voltage drop (i.e., an IR drop) occurs by resistance of a line and/or an electrode. Here, in the case of the top emission type organic light emitting display, the voltage drop is caused by the high resistance of the thin, transflective cathode electrode, making it difficult to properly implement the display.
In other words, a cathode electrode having a high resistance can result in a serious luminance non-uniformity in the inside of the panel and, particularly in large-area organic light emitting displays, a voltage drop (IR drop) occurs between the center portion and the outer portion of the panel, restricting current injection for high luminance emission in a high definition display.